1. Field of the Invention
The present invention relates to a method of broadening the dynamic range of a solid-state image sensor included in a digital still camera or similar image pickup apparatus.
2. Description of the Background Art
It is a common practice with a CCD (Charge Coupled Device) image sensor or similar solid-state image sensor to broaden a dynamic range by preparing two different images produced by photosensitive cells different in photosensitivity from each other. An image derived from high-sensitivity photosensitive cells appear relatively clear even when a subject is relatively dark, because those cells respond even to a small amount of incident light. However, if lightness of a desired subject exceeds preselected luminance, then the high-sensitivity photosensitive cells are saturated and render the image white and unclear. On the other hand, an image derived from low-sensitivity photosensitive cells render even a light subject, which would cause the high-sensitivity cells to be saturated, clear although rendering a dark subject even darker and unclear. Therefore, by combining the two images each having a particular advantage as stated above, it is possible to broaden the dynamic range for rendering any kind of subject clear regardless of the luminance distribution of a scene. This kind of scheme is taught in, e.g. Hiroo Takemura “Introduction to CCD Camera Technologies”, First Edition, CORONA PUBLISHING CO., LTD., Dec. 1997, pp. 100-102.
However, it has been customary with the solid-state image sensor of the type having partitioned photosensitive cells, each having a main region and a subregion. According to this type of image sensor, signal charges are read out sequentially from the main regions and then from the subregions, taking a period of time corresponding to two fields of image.